Heat sinks are provided in conjunction with electronic device packages to conduct heat generated by the electronic device during use and to dissipate the heat to the environment so that the electronic device will not be damaged or its performance affected by the heat. Such electronic device packages include transistors, diodes, resistors and the like which are typically connected to a circuit through mounting on a printed circuit board. Generally, heat sinks are constructed of a material, usually a metal such as aluminum, having a high coefficient of thermal conduction and are formed into shapes adapted to convect or radiate heat at a high rate. In order for the heat sink to operate efficiently, it must be secured to and placed in good thermal contact with the electronic device package.
Various means are used to accomplish this purpose. For instance, it is known to provide an aperture in the electronic device package and to secure it to the heat sink by a screw, bolt or other mechanical means. However, this arrangement is somewhat time consuming. It may also be desirable to periodically or temporarily remove the heat sink, such as if the electronic device package to which the heat sink is secured is being replaced or repaired. For this purpose heat sinks have been developed having resilient spring clips which frictionally engage the electronic device package yet allow the heat sink to be quickly and easily attached or removed from the electronic device package. Such clips may be integrally formed with the heat sink or may be constructed separately and then bonded to the heat sink. It is to the latter type of heat sink that the present invention is directed.
It is also common and desirable to apply various surface treatments to the exterior of the heat sink and clip after assembly. Such surface treatments include, but are not limited to, anodization, electrodeposition, diffusion coating, galvanization, cladding, sprayed metal or paint coatings and conversion coatings as well as various forms of enamels, plastics, rubbers and vacuum deposited coatings. These treatments are beneficial in that they protect the heat sink and clip from corrosion caused by moisture or other materials in the environment. Although the heat sink and the clip may be surface treated separately before bonding, the bonding process tends to destroy the surface treatment beneath and adjacent the location of the bonding, exposing the unprotected material of the heat sink and permitting localized corrosion to occur. If the corrosion is severe enough, the effectiveness of the heat sink in transferring heat from the electronic device package may be reduced, the heat sink may be damaged or destroyed, or the integrity of the bonding of the clip to the heat sink may be reduced. In addition, certain of the surface treatments described above enable the heat sink to be selectively colored. Darker colors are desirable in that they enhance the ability of the heat sink to radiate heat to the atmosphere during use.
A conventional design for mounting the clip to the heat sink body is shown in FIGS. 1 and 2. Heat sink 10 includes flat middle body portion 12 with opposing flange portions 14 and 16, each having a C-shaped cross-section. Each of the flange portions also include a plurality of transverse slots 18 formed therein, which construction is known in the art to be effective in dissipating heat to the atmosphere through convection and radiation. Clip 20 includes attachment portion 22, concave middle portion 24 and lip 26. The attachment portion of the clip is bonded on the base of the heat sink at location 28 such as by spot welding or ultrasonic bonding. The clip is adapted to resiliently grip an electronic device package inserted into the gap between the concave portion of the clip and the base portion of the heat sink, guided by the lip.
Although the attachment portion of the clip is intended to present a flat parallel surface to the base portion of the heat sink for bonding purposes, there are inevitably irregularities and deformations in the attachment portion of the clip. Further, similar irregularities and deformations are commonly found in base portion of the heat sink. Therefore, the various surface irregularities and deformations create random gaps therebetween of varying thickness and area, as illustrated at 30 in FIG. 2, or areas of contact when the clip is bonded on the heat sink. This has the undesirable effect that when the heat sink and clip are surface treated after assembly, portions of the opposing surfaces on each part may not be accessible and thus not effectively treated. Most surface treatment processes require that the surfaces to be treated be prepared by removing all oils, greases, dirt, dust, metal cuttings, oxide scales, cutting or grinding fluids or other contaminants. The surface is then cleaned again with a solvent and etched with an acid such as sulfuric acid or hydrochloric acid. Finally, the surfaces are sealed beneath a protective layer to shield them from corrosive elements. For instance, if aluminum is being anodized, a layer of aluminum oxide is created by an electropheresis process. If the surfaces are not adequately prepared, the surface treatment will not be completely effective and subsequent exposure of the heat sink to a harsh environment may induce corrosion on the exposed surfaces. Further, the bonding between the heat sink and the clip may not be complete or be adversely affected by subsequent corrosion, thus reducing the reliability with which the heat sink is secured to the electronic device package. It is thus desirable to construct a heat sink with a clip which is securely bonded on the heat sink, but in which all exposed surfaces are accessible after assembly for surface treatment.